This invention relates to vehicle drive systems and more particularly to systems for coordinating the operation of separate transmission systems in vehicles having more than one engine.
To provide increased traction and load-carrying capacity, certain vehicles, such as large earthmoving equipment, are provided with more than one engine in an arrangement wherein each engine drives a separate set of wheels, tracks or other ground-engaging members through separate transmissions. Under most conditions, it is desirable that the drive ratios of the transmissions be the same at any given time. If the drive ratios are dissimilar, a portion of the total engine output is unused and wear of various vehicle components is accelerated. Typically, the drive ratio is manually or automatically selected at one transmission, which may be termed the master transmission, and remote control means are provided to force a corresponding shift of the other transmission, which may be termed the slave transmission, whenever the master transmission is shifted.
Commonly, the control of the slave transmission is accomplished by use of a rotary switch coupled to the shift control for the master transmission, the switch having a plurality of positions corresponding to each of the drive ranges of the transmission. As the transmission is shifted from one drive range to another, the switch is moved accordingly. The switch positions are connected, by means of a multi-conductor cable, to relays at the shift control for the slave transmission. As the switch changes position, an appropriate relay is energized to cause the rear transmission to shift.
Such systems have a number of disadvantages. Battery power has to be carried through the connecting cable in order to actuate the remotely located relays, and power consumption is accordingly increased. The drive-range-indicating switch also has to handle the relatively high battery current to the relays, thus presenting arcing problems at the switch contacts as a circuit is broken, which leads to premature failure. If the switch resistance is increased, as by dirty contacts, the high current in the circuit will produce an undesirably large voltage drop at the contacts.
Additionally, the connecting cable is usually subject to physical abuse and may develop faults such that a wrong relay is energized in response to the master transmission switch position and the slave transmission is shifted to an improper drive range.